In order to control multiple joints of an industrial robot, an upper level planning controller provides commands to one or more PID controllers. A PID controller is provided for each joint of the robot and is coupled to an actuator, such as a motor or the like, that controls movement of the joint. The PID controller may not be programmed as a compliant controller that provides both position control and force control in response to commands from the planning controller.
Known compliant controllers provide position control and force control for a joint or actuator by switching between a position control algorithm and a force control algorithm as needed. Heretofore, such complaint controllers have not been able to simultaneously provide position and force control in the same direction. Typically, with these known controllers, if a joint is to move in the X direction and apply a force in the X direction, the position control algorithm would be used to move the joint to a desired or commanded position, at which point the movement of the joint would be stopped after a controlled deceleration and the controller would switch to the force control algorithm. In order to control force, known compliant controllers require force sensors to monitor the forces encountered by the actuator so that the force control can provide the appropriate response. During position control, if an unexpected, i.e., disturbing force is encountered, known position controllers respond by increasing the output torque of the actuator in an attempt to overcome the disturbing force and reach the desired position. This can result in damage to the actuator. In attempting to overcome this problem, complex software programs have been developed to control the switching between the position and force controls and to account for the decelerations, etc., associated with stopping movement in order to switch. Because these controllers are so complex and are not sufficiently adaptable to a wide range of disturbing forces, they have not been widely adopted in industry.
As an alternative to known types of compliant controllers discussed above, research has been conducted to control robot joints based on a biological model such as a primate muscle. This research has also extended into using the biological model of a primate muscle for active suspension control. This research is described in the following articles. Wu, C. H., J. C. Houk, K. Y. Young, and L. E. Miller, "Nonlinear Damping of Limb Motion" book chapter, Multiple Muscle Systems: Biomechanics and Movement Organization edited by J. M. Winters and S. L-Y. Woo, Springer-Verlag New York Publishers, 1990, pp. 214-235. Wu, C. H., K. Y. Young, and J. C. Houk, "A neuromuscular-Like Model for Robotic Compliance Control," Proceedings of 1990 IEEE Int. Conference on Robotics and Automation, Cincinnati, Ohio, May 1990, pp. 1885-1890. Wu, C. H., K. Y. Young, and K. S. Hwang, "Analysis of Voluntary Movements for Robotic Control," IEEE 1991 International Conference on Robotics and Automation, Sacramento, Calif., Apr. 1991. Wu, C. H., K. Y. Young, K. S. Hwang, and S. Lehman, "Voluntary Movements for Robotic Control," IEEE Control Systems Magazine, Vol. 2, No. 1, February 1992, pp. 8-14. Wu, C. H. and K. S. Hwang, "Nonlinear Neuromuscular Control for Robotic Compliance Control," Proceedings of the 1993 IEEE International Symposium on Intelligent Control, August 1993, 238-243. Wu, C. H. and S. L. Chang, "Implementation of a Neuromuscular-like Control for Compliance on A PUMA 560 Robot," Proceedings of the 34.sup.th IEEE International Conference on Decision and Control, New Orleans, La., Dec. 1995, 1597-1602. Wu, C. H., S. L. Chang, and D. T. Lee, "A Study of Neuromuscular-like Control for Rehabilitation Robots," Proceedings of the 1996 IEEE International Conference on Robotics and Automation, Minneapolis, Minn., April 1996. Chang, S.L. and C.H. Wu, "Design of an Active Suspension System Based on a Biological Model," Proceedings of 1997 American Control Conference, Albuquerque, N. Mex., June 1997. Wu, C. H., K. S. Hwang, and S. L. Chang, "Analysis and Implementation of a Neuromuscular-like Control for Robotic Compliance," IEEE Transactions on Control Systems Technology, Vol. 5, No. 6, November 1997, pp. 586-597. However, the implementation of the biological model to form a compliant controller suitable for commercialization has been a complex task.